Diffusers can be used to improve the acoustics of enclosed spaces to make music more beautiful and speech more intelligible. Early research in diffusers began by considering non-absorbing reflection phase grating surfaces such as Schroeder diffusers. These surfaces consist of a series of wells of the same width and different depths. The wells are separated by thin dividers. The depths of the wells are determined by a mathematical number theory sequence that has a flat power spectrum such as a quadratic residue or primitive root sequence. More recent research has concerned the development of “diffsorbers” or hybrid absorber-diffusers; these are surfaces that are combinations of amplitude and phase gratings, where partial absorption is inherent in the design, and any reflected sound is dispersed.
A diffuser needs to break up the reflected wavefront. While this can be achieved by shaping a surface, as in a phase grating, it can also be achieved by changing the impedance of the surface. In hybrid surfaces, variable impedance is achieved by patches of absorption and reflection, giving pressure reflection coefficients nominally of 0 and 1, respectively. Unlike the Schroeder diffuser, these cannot be designed for minimum absorption. These surfaces are hybrids, somewhere between pure absorbers and non-absorbing diffusers.
The use of patches of absorption to generate dispersion is not particularly new. In studio spaces, people have been arranging absorption in patches rather than solid blocks for many years. In recent times, however, a new breed of surface has been produced, where the absorbent patches are much smaller, and the arrangement of these patches is determined by a pseudorandom sequence to maximize the dispersion generated. For instance, the Binary Amplitude Diffsorber, also known as a BAD panel, assigned to Applicants' Assignee, is a flat hybrid surface having both absorbing and diffusing abilities with the location of the absorbent patches determined by a Maximum Length Sequence (MLS). The panel simultaneously provides sound diffusion at high and mid b and frequencies, and crosses over to absorption below some cut-off frequency. In FIG. 1, a simple binary amplitude diffuser, based on an N=7 maximum length sequence {1110010}, is depicted. The white patches are made of hard material and are reflecting with a pressure reflection coefficient of 1 and the shaded patches are made of absorbent material and so are absorbing with a pressure reflection coefficient of 0. By changing the number of hard and soft patches on the surface, it is possible to control the absorption coefficient. By changing the ordering of the patches, it is possible to control how the reflected sound is distributed. If a periodic arrangement of patches is used, then the reflected sound will get concentrated in particular directions due to spatial aliasing; these are then grating lobes. If a good pseudo-random sequence is used to choose the patch order (say a Barker sequence), then the scattering will be more even. Applicants have described in U.S. Pat. No. 5,817,992 effective planar two-dimensional binary amplitude sequences.
A problem with planar hybrid absorber-diffusers is that energy can only be removed from the specular reflection by absorption. While there is diffraction caused by the impedance discontinuities between the hard and soft patches, this is not a dominant mechanism except at low frequencies. Even with the most optimal arrangement of patches, at high frequencies where the patch becomes smaller than half the wavelength, the specular reflection is only attenuated by roughly 7 dB, for a surface with 50% absorptive area, because 3/7ths of the surface forms a flat plane surface, which reflects unaltered by the presence of the absorptive patches.
If it were possible to exploit interference, by reflecting waves out-of-phase with the specular lobe, then it would be possible to diminish the specular lobe further.
Applicants have found that this can be achieved by using a new class of hybrid diffusers combining the aspects of an amplitude grating with those of a reflection phase grating. These new surfaces contain the elements of an amplitude grating, namely, reflective and absorptive patches, with the addition of a additional reflective patches, in the form of wells a quarter wavelength deep at the design frequency, which can constructively interfere with the zero-depth reflective patches. The simplest form of these hybrid gratings is an absorber-diffuser with a random or pseudo-random distribution. But a more effective design is based on a ternary sequence, which nominally has surface reflection coefficients of 0, 1 and −1. The wells with the pressure reflection coefficient of −1 typically have a depth of a quarter of a wavelength at the design frequency and odd multiples of this frequency to produce waves out of phase with those producing the specular lobe, i.e. the wells with a pressure reflection coefficient of +1. This results in a better reduction of the specular reflection. By contrast with the N=7 binary sequence {1110010} with three purely reflective elements, which offers 7 dB [20*log ( 3/7)] of specular attenuation, an N=7 ternary sequence {1 1 0 1 0 0 −1} with two remaining purely reflective elements due to cancellation of a 1 and −1, offers 11 dB [20*log ( 2/7)] of attenuation. Ternary sequences are therefore an extension of the binary amplitude diffuser and are an alternative way of forming hybrid absorber-diffusers, which achieve superior scattering performance for a similar amount of absorption, as the BAD panel. As will be described, there are other sequences and approaches, using both single plane and hemispherically scattering designs.